1. Field of the Invention
The present invention relates to an oxygen sensor control apparatus which calibrates the relation between the oxygen concentration of exhaust gas discharged from an internal combustion engine and the output of an oxygen sensor for detecting the oxygen concentration of the exhaust gas and which detects the oxygen concentration of the exhaust gas.
2. Description of the Related Art
Conventionally, an oxygen sensor has been disposed in an exhaust passage (exhaust pipe) of an internal combustion engine of an automobile or the like so as to detect the concentration of oxygen contained in exhaust gas for the purpose of controlling the air-fuel ratio of a fuel mixture supplied to the engine. Such an oxygen sensor includes, for example, a gas detection element which has at least one cell composed of an oxygen ion conductive zirconia substrate and a pair of electrodes formed thereon. However, such an oxygen sensor has a problem. That is, accuracy in detecting the oxygen concentration is inconsistent because of variation in output characteristics among individual oxygen sensors and deterioration of each oxygen sensor with time. In order to solve such a problem, a technique of performing atmospheric correction has been proposed; i.e., a technique of stopping supply of fuel to an internal combustion engine, and calibrating the relation between the output of the oxygen sensor and oxygen concentration when the exhaust passage is assumed to be substantially completely filled with atmospheric air (see, for example, Patent Document 1).    [Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2007-32466 (paragraph 0040)
3. Problems to be Solved by the Invention:
However, the atmospheric correction method described in Patent Document 1 merely calculates a correction coefficient by comparing a reference output value Vstd which is output from a standard oxygen sensor in the atmospheric air and a current output value (i.e., a single output value) Vsen of an oxygen sensor in a fuel cut period during which the supply of fuel to the internal combustion engine is stopped. Even in such a fuel cut period, the output value of the oxygen sensor fluctuates because it pulsates as a result of operation of the internal combustion engine, and/or noise is superimposed on the output thereof. Therefore, the method of calculating the correction coefficient by merely comparing one output value of the oxygen sensor during a fuel cut period with the reference output value has a problem in that obtaining an accurate correction coefficient is difficult.